With the drastic increase of human population, there is an ever-growing demand for energy and clean water for the continuous economic growth and suitable inhabitation on earth. Millions of tons of wastewater are produced from industrial and agricultural operations each year, and about 25 billion U.S. dollars are spent annually for wastewater treatment in the United States alone. It is highly desirable to employ energy-efficient processes for wastewater treatment and simultaneously recover the energy contained as organic matter in wastewater. It has been thought that this can possibly be achieved by microbial fuel cell (MFC) technology. MFCs are bioelectrochemical devices, where electrogenic bacteria are used to oxidize the organic matter, transfer the electrons to an electrode, and generate electrical energy. In addition to bioelectricity, the electrons produced by the microorganisms can also be used to produce various chemical fuels, depending on the electron acceptors used in the catholyte.
When protons serve as terminal electron acceptors, hydrogen gas will be produced at the cathode. While the microbial electrohydrogenesis process has been experimentally demonstrated using a wide range of microorganisms with various organic nutrients, thermodynamics constraints limit microbial electrogenesis and hydrogen production occurs simultaneously without the addition of an external bias. To overcome the thermodynamic constraints and to compensate for the energy loss during the operation (e.g., due to electrical resistance of the device), an external bias of 0.2 to 1.0 V is needed to sustain the current/hydrogen generation. Nevertheless, the requirement of external bias adds to the complexity and cost for hydrogen production, prohibiting it as a cost effective energy solution. Considerable efforts have been made to minimize the energy loss through the optimization of MFC reactors, electrodes, as well as the type of metal catalysts on the cathode. It has also been reported that microbial electrohydrogenesis can be driven by a solar cell or another MFC.
What is needed is a device that produces bioelectricity at zero external potential, which can generate hydrogen gas at zero external bias using biodegradable organic matter and solar light as the only energy sources.